diff --git a/libusb/core.c b/libusb/core.c
index bac340b..e103724 100644
--- a/libusb/core.c
+++ b/libusb/core.c
@@ -700,16 +700,11 @@
 {
 	size_t priv_size = usbi_backend.device_priv_size;
 	struct libusb_device *dev = calloc(1, PTR_ALIGN(sizeof(*dev)) + priv_size);
-	int r;
 
 	if (!dev)
 		return NULL;
 
-	r = usbi_mutex_init(&dev->lock);
-	if (r) {
-		free(dev);
-		return NULL;
-	}
+	usbi_mutex_init(&dev->lock);
 
 	dev->ctx = ctx;
 	dev->refcnt = 1;
@@ -1267,11 +1262,7 @@
 	if (!_dev_handle)
 		return LIBUSB_ERROR_NO_MEM;
 
-	r = usbi_mutex_init(&_dev_handle->lock);
-	if (r) {
-		free(_dev_handle);
-		return LIBUSB_ERROR_OTHER;
-	}
+	usbi_mutex_init(&_dev_handle->lock);
 
 	r = usbi_backend.wrap_sys_device(ctx, _dev_handle, sys_dev);
 	if (r < 0) {
@@ -1325,11 +1316,7 @@
 	if (!_dev_handle)
 		return LIBUSB_ERROR_NO_MEM;
 
-	r = usbi_mutex_init(&_dev_handle->lock);
-	if (r) {
-		free(_dev_handle);
-		return LIBUSB_ERROR_OTHER;
-	}
+	usbi_mutex_init(&_dev_handle->lock);
 
 	_dev_handle->dev = libusb_ref_device(dev);
 
diff --git a/libusb/io.c b/libusb/io.c
index ebad383..b625620 100644
--- a/libusb/io.c
+++ b/libusb/io.c
@@ -1793,7 +1793,7 @@
 	}
 
 	r = usbi_mutex_trylock(&ctx->events_lock);
-	if (r)
+	if (!r)
 		return 1;
 
 	ctx->event_handler_active = 1;
@@ -2016,11 +2016,10 @@
 
 	r = usbi_cond_timedwait(&ctx->event_waiters_cond,
 		&ctx->event_waiters_lock, tv);
-
 	if (r < 0)
-		return r;
-	else
-		return (r == ETIMEDOUT);
+		return r == LIBUSB_ERROR_TIMEOUT;
+
+	return 0;
 }
 
 static void handle_timeout(struct usbi_transfer *itransfer)
diff --git a/libusb/os/threads_posix.c b/libusb/os/threads_posix.c
index 071e205..d79e8a6 100644
--- a/libusb/os/threads_posix.c
+++ b/libusb/os/threads_posix.c
@@ -21,6 +21,7 @@
 
 #include "libusbi.h"
 
+#include <errno.h>
 #if defined(__ANDROID__)
 # include <unistd.h>
 #elif defined(__HAIKU__)
@@ -55,15 +56,22 @@
 		timeout.tv_sec++;
 	}
 
-	return pthread_cond_timedwait(cond, mutex, &timeout);
+	r = pthread_cond_timedwait(cond, mutex, &timeout);
+	if (r == 0)
+		return 0;
+	else if (r == ETIMEDOUT)
+		return LIBUSB_ERROR_TIMEOUT;
+	else
+		return LIBUSB_ERROR_OTHER;
 }
 
-int usbi_get_tid(void)
+unsigned int usbi_get_tid(void)
 {
-	static _Thread_local int tid;
+	static _Thread_local unsigned int tl_tid;
+	int tid;
 
-	if (tid)
-		return tid;
+	if (tl_tid)
+		return tl_tid;
 
 #if defined(__ANDROID__)
 	tid = gettid();
@@ -101,5 +109,5 @@
 		tid = (int)(intptr_t)pthread_self();
 	}
 
-	return tid;
+	return tl_tid = (unsigned int)tid;
 }
diff --git a/libusb/os/threads_posix.h b/libusb/os/threads_posix.h
index eadb978..e187c69 100644
--- a/libusb/os/threads_posix.h
+++ b/libusb/os/threads_posix.h
@@ -23,63 +23,69 @@
 
 #include <pthread.h>
 
+#define PTHREAD_CHECK(expr)			\
+	do {					\
+		int pthread_result = (expr);	\
+		assert(pthread_result == 0);	\
+	} while (0)
+
 #define USBI_MUTEX_INITIALIZER	PTHREAD_MUTEX_INITIALIZER
 typedef pthread_mutex_t usbi_mutex_static_t;
 static inline void usbi_mutex_static_lock(usbi_mutex_static_t *mutex)
 {
-	(void)pthread_mutex_lock(mutex);
+	PTHREAD_CHECK(pthread_mutex_lock(mutex));
 }
 static inline void usbi_mutex_static_unlock(usbi_mutex_static_t *mutex)
 {
-	(void)pthread_mutex_unlock(mutex);
+	PTHREAD_CHECK(pthread_mutex_unlock(mutex));
 }
 
 typedef pthread_mutex_t usbi_mutex_t;
-static inline int usbi_mutex_init(usbi_mutex_t *mutex)
+static inline void usbi_mutex_init(usbi_mutex_t *mutex)
 {
-	return pthread_mutex_init(mutex, NULL);
+	PTHREAD_CHECK(pthread_mutex_init(mutex, NULL));
 }
 static inline void usbi_mutex_lock(usbi_mutex_t *mutex)
 {
-	(void)pthread_mutex_lock(mutex);
+	PTHREAD_CHECK(pthread_mutex_lock(mutex));
 }
 static inline void usbi_mutex_unlock(usbi_mutex_t *mutex)
 {
-	(void)pthread_mutex_unlock(mutex);
+	PTHREAD_CHECK(pthread_mutex_unlock(mutex));
 }
 static inline int usbi_mutex_trylock(usbi_mutex_t *mutex)
 {
-	return pthread_mutex_trylock(mutex);
+	return pthread_mutex_trylock(mutex) == 0;
 }
 static inline void usbi_mutex_destroy(usbi_mutex_t *mutex)
 {
-	(void)pthread_mutex_destroy(mutex);
+	PTHREAD_CHECK(pthread_mutex_destroy(mutex));
 }
 
 typedef pthread_cond_t usbi_cond_t;
 static inline void usbi_cond_init(pthread_cond_t *cond)
 {
-	(void)pthread_cond_init(cond, NULL);
+	PTHREAD_CHECK(pthread_cond_init(cond, NULL));
 }
 static inline void usbi_cond_wait(usbi_cond_t *cond, usbi_mutex_t *mutex)
 {
-	(void)pthread_cond_wait(cond, mutex);
+	PTHREAD_CHECK(pthread_cond_wait(cond, mutex));
 }
 int usbi_cond_timedwait(usbi_cond_t *cond,
 	usbi_mutex_t *mutex, const struct timeval *tv);
 static inline void usbi_cond_broadcast(usbi_cond_t *cond)
 {
-	(void)pthread_cond_broadcast(cond);
+	PTHREAD_CHECK(pthread_cond_broadcast(cond));
 }
 static inline void usbi_cond_destroy(usbi_cond_t *cond)
 {
-	(void)pthread_cond_destroy(cond);
+	PTHREAD_CHECK(pthread_cond_destroy(cond));
 }
 
 typedef pthread_key_t usbi_tls_key_t;
 static inline void usbi_tls_key_create(usbi_tls_key_t *key)
 {
-	(void)pthread_key_create(key, NULL);
+	PTHREAD_CHECK(pthread_key_create(key, NULL));
 }
 static inline void *usbi_tls_key_get(usbi_tls_key_t key)
 {
@@ -87,13 +93,13 @@
 }
 static inline void usbi_tls_key_set(usbi_tls_key_t key, void *ptr)
 {
-	(void)pthread_setspecific(key, ptr);
+	PTHREAD_CHECK(pthread_setspecific(key, ptr));
 }
 static inline void usbi_tls_key_delete(usbi_tls_key_t key)
 {
-	(void)pthread_key_delete(key);
+	PTHREAD_CHECK(pthread_key_delete(key));
 }
 
-int usbi_get_tid(void);
+unsigned int usbi_get_tid(void);
 
 #endif /* LIBUSB_THREADS_POSIX_H */
diff --git a/libusb/os/threads_windows.c b/libusb/os/threads_windows.c
index cf72694..4a57f42 100644
--- a/libusb/os/threads_windows.c
+++ b/libusb/os/threads_windows.c
@@ -34,7 +34,7 @@
 	if (SleepConditionVariableCS(cond, mutex, millis))
 		return 0;
 	else if (GetLastError() == ERROR_TIMEOUT)
-		return ETIMEDOUT;
+		return LIBUSB_ERROR_TIMEOUT;
 	else
-		return EINVAL;
+		return LIBUSB_ERROR_OTHER;
 }
diff --git a/libusb/os/threads_windows.h b/libusb/os/threads_windows.h
index 618c2f3..ed57f7f 100644
--- a/libusb/os/threads_windows.h
+++ b/libusb/os/threads_windows.h
@@ -21,7 +21,11 @@
 #ifndef LIBUSB_THREADS_WINDOWS_H
 #define LIBUSB_THREADS_WINDOWS_H
 
-#include <errno.h>
+#define WINAPI_CHECK(expr)			\
+	do {					\
+		BOOL winapi_result = (expr);	\
+		assert(winapi_result != 0);	\
+	} while (0)
 
 #define USBI_MUTEX_INITIALIZER	0L
 typedef LONG usbi_mutex_static_t;
@@ -36,10 +40,9 @@
 }
 
 typedef CRITICAL_SECTION usbi_mutex_t;
-static inline int usbi_mutex_init(usbi_mutex_t *mutex)
+static inline void usbi_mutex_init(usbi_mutex_t *mutex)
 {
 	InitializeCriticalSection(mutex);
-	return 0;
 }
 static inline void usbi_mutex_lock(usbi_mutex_t *mutex)
 {
@@ -51,14 +54,13 @@
 }
 static inline int usbi_mutex_trylock(usbi_mutex_t *mutex)
 {
-	return !TryEnterCriticalSection(mutex);
+	return TryEnterCriticalSection(mutex) != 0;
 }
 static inline void usbi_mutex_destroy(usbi_mutex_t *mutex)
 {
 	DeleteCriticalSection(mutex);
 }
 
-// We *were* getting timespec from pthread.h:
 #if !defined(HAVE_STRUCT_TIMESPEC) && !defined(_TIMESPEC_DEFINED)
 #define HAVE_STRUCT_TIMESPEC 1
 #define _TIMESPEC_DEFINED 1
@@ -68,11 +70,6 @@
 };
 #endif /* HAVE_STRUCT_TIMESPEC || _TIMESPEC_DEFINED */
 
-// We *were* getting ETIMEDOUT from pthread.h:
-#ifndef ETIMEDOUT
-#define ETIMEDOUT	10060	/* This is the value in winsock.h. */
-#endif
-
 typedef CONDITION_VARIABLE usbi_cond_t;
 static inline void usbi_cond_init(usbi_cond_t *cond)
 {
@@ -80,7 +77,7 @@
 }
 static inline void usbi_cond_wait(usbi_cond_t *cond, usbi_mutex_t *mutex)
 {
-	(void)SleepConditionVariableCS(cond, mutex, INFINITE);
+	WINAPI_CHECK(SleepConditionVariableCS(cond, mutex, INFINITE));
 }
 int usbi_cond_timedwait(usbi_cond_t *cond,
 	usbi_mutex_t *mutex, const struct timeval *tv);
@@ -97,6 +94,7 @@
 static inline void usbi_tls_key_create(usbi_tls_key_t *key)
 {
 	*key = TlsAlloc();
+	assert(*key != TLS_OUT_OF_INDEXES);
 }
 static inline void *usbi_tls_key_get(usbi_tls_key_t key)
 {
@@ -104,16 +102,16 @@
 }
 static inline void usbi_tls_key_set(usbi_tls_key_t key, void *ptr)
 {
-	(void)TlsSetValue(key, ptr);
+	WINAPI_CHECK(TlsSetValue(key, ptr));
 }
 static inline void usbi_tls_key_delete(usbi_tls_key_t key)
 {
-	(void)TlsFree(key);
+	WINAPI_CHECK(TlsFree(key));
 }
 
-static inline int usbi_get_tid(void)
+static inline unsigned int usbi_get_tid(void)
 {
-	return (int)GetCurrentThreadId();
+	return (unsigned int)GetCurrentThreadId();
 }
 
 #endif /* LIBUSB_THREADS_WINDOWS_H */
diff --git a/libusb/version_nano.h b/libusb/version_nano.h
index 41b9eb4..65fa0d7 100644
--- a/libusb/version_nano.h
+++ b/libusb/version_nano.h
@@ -1 +1 @@
-#define LIBUSB_NANO 11554
+#define LIBUSB_NANO 11555